1. Field
The present invention relates to a method for separating an epitaxial layer from a growth substrate.
2. Discussion of the Background
A light emitting diode (LED) is an optical semiconductor device using a principle that when a voltage is applied to a junction of a P-type semiconductor and an N-type semiconductor, energy released upon recombination of electrons in an N-region with holes in a P-region is emitted as light. Such LEDs have characteristics of environmental friendliness, low driving voltages, long lifespan, low costs, etc. Conventionally, the LEDs have been frequently applied to lamps for display or to display of simple information such as numbers. However, with development of industrial technologies, particularly information display and semiconductor technologies, the LEDs have been recently used in various fields such as displays, vehicle headlamps and projectors.
Particularly, an LED using a Group III-V based semiconductor such as gallium nitride (GaN) has a direct transition energy band structure, so that it has an advantage of high internal quantum efficiency. Accordingly, the GaN semiconductor has recently come into the spotlight as a material for LEDs.
In a fabrication of the LED, it is difficult to make a homogeneous substrate on which an epitaxial layer of the LED can be grown. Therefore, the epitaxial layer is grown on a heterogeneous growth substrate having a similar crystal structure by means of a process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Generally, a sapphire substrate with a hexagonal system is mainly used as a growth substrate. However, since the sapphire substrate is electrically non-conductive, it restricts the structure of an LED formed thereon.
Accordingly, studies have been conducted to develop a technique for fabricating a vertical LED by growing epitaxial layers on a growth substrate such as a sapphire substrate and then separating the growth substrate.
A method for removing a substrate by means of a substrate polishing process may be used as a method for separating the growth substrate. However, the removal of the growth substrate by polishing the growth substrate requires much time and high costs.
Therefore, a laser lift-off (LLO) method, a stress lift-off (SLO) method or a chemical lift-off (CLO) method is mainly used as the method for separating the epitaxial layer from the growth substrate.
Here, the LLO method is a method in which an epitaxial layer is grown on a growth substrate, a support substrate is bonded onto an epitaxial layer, and a laser beam then irradiates through the growth substrate to separate the epitaxial layer from the growth substrate. The SLO method is a method in which a convexo-concave pattern is formed on one surface of a growth substrate, a passivation layer such as an insulation film is formed on some regions of the growth substrate such that an epitaxial layer is grown on only other regions of the growth substrate, and a thick epitaxial layer is grown and then cooled to separate the epitaxial layer from the growth substrate due to surface stress. The CLO method is a method in which a pattern is formed by using a material, which is susceptible to chemical damage, on a surface of a growth substrate, an epitaxial layer is grown, and the material that is susceptible to chemical damage is electrochemically or chemically removed to separate the epitaxial layer from the growth substrate.
However, in the LLO method among the aforementioned methods for separating the growth substrate, heat generated by irradiation of a laser beam has influence on the epitaxial layer, thereby deteriorating characteristics of the epitaxial layer. In the SLO method or CLO method, a separate process of processing the surface of the growth substrate is required before the epitaxial layer is grown, which results in complicated processes as well as problems with mass production due to much time that is taken to separate the epitaxial layer. In the SLO method, the epitaxial layer can be separated without damage thereto only when it is grown to be thick, and thus, it is not easy to apply this method to actual processes.